This invention relates to line transducers and more particularly to an improved electret line transducer. One application of the invention is in an intrusion detection system.
The coaxial cable transducer described in U.S. Pat. No. 3,763,482 has been used successfully to detect mechanical disturbances over a fairly wide range of frequencies, for example, from 1 Hz to 10 KHz. An example of an application in which disturbances in this frequency range are efficiently detected by the coaxial cable transducer is a chain link fence to which the transducer is tightly secured by ties as described in the foregoing patent. The sensitivity of the coaxial cable transducer, however, diminishes at lower frequencies such as 0.1 Hz - 0.5 Hz because of the cylindrical shape of the outer conductor. When the outer conductor is exposed to each low-frequency unidirectional compression, the spacing between the outer conductor and the outer surface of the electret filler decreases at some locations but increases at others so as to have a net signal cancellation effect. This results in a reduced sensitivity of the transducer.
Another consideration of the coaxial cable configuration is its relatively high cost compared to twin line cable, the latter being about 1/2 to 1/3 of the cost of the coaxial cable. It would be advantageous, therefore, from a cost standpoint to utilize a twin lead cable as a substitute for the coaxial cable in line transducer applications.